V43G-01 INVITED
Zircon U-Th and U-Pb Ages From Quaternary Silicic Volcanic and Plutonic Rocks, and Their Bearing on Granitoid Batholiths
In the ten years since publication of M. Reid et al.'s seminal paper on zircon ages from rhyolites (EPSL 150:2-39, 1997) >20 papers have appeared on SIMS 238U-230Th and 238U-206Pb geochronology of zircon from silicic volcanic rocks, plutonic xenoliths, and young intrusions. In some cases, as well as for U-Pb studies of Tertiary granitoids, plutonic samples are interpreted in the context of related volcanism. These geochronologic data have advanced conceptual models of silicic magma genesis and pluton construction. Of fundamental importance are discoveries that zircons in volcanic rocks typically pre-date eruption by 10's to 100's of kyr and that multiple zircon populations are common; these crystals are "antecrysts" recycled from intrusive rocks or crystal mush of the system that vented. Resolving such age differences is possible with U-Th at <300 ka but is challenging with U-Pb, where SIMS precision limits resolution of differences on the order of 100 kyr for Pleistocene-Miocene zircons. Cathodoluminescence (CL) imaging of polished crystals guides beam placement but leads to sampling bias that favors high-U regions. Thus, although model-age histograms and relative probability plots identify zircon age populations, they are unlikely to accurately define relative abundances of age groups. Microbeam analysis collects data for the entire volume sampled but only SIMS depth-profiling into crystal faces can spatially resolve fine zones. ID-TIMS analysis of CL-imaged zircon fragments can improve U-Pb precision. SIMS complements geochronology with trace element fingerprints of zircon growth environments and enables Ti-in-zircon thermometry. Literature examples illustrate recent findings: (1) rhyodacite lava at Crater Lake contains zircons derived from late Pleistocene granodiorite represented by blocks ejected in the caldera-forming eruption; (2) zircons in Mount St. Helens dacites grew at sub-eruption temperatures and pre-date eruptions by up to 250 kyr; (3) Miocene plutons near Mount Rainier and the Colorado River were emplaced and crystallized in pulses over ~2-3-Myr periods, some with coeval volcanics; and (4) Cretaceous batholiths in the Sierra Nevada and North Cascades preserve evidence of assembly over as much as 10 Myr; individual samples contain zircons that crystallized during intervals of >1 Myr. Zircon ages and wide-ranging trace element concentrations suggest crystallization mainly in differentiated melt pockets in high-crystallinity magmas that may repeatedly freeze and thaw. Some high-Th/U, incompatible-element rich, spongy textured zircons grew very late, in the presence of oxidizing fluid. Not all zircons survive recycling into undersaturated magmas, in which zircon will dissolve given enough time, depending on temperature and dissolved volatiles. Recent zircon geochronologic results for volcanic and plutonic rocks lend credence to the "mush model" of rhyolite genesis and batholith consolidation. Crystal-poor rhyolites and leucogranites are melts segregated by compaction or gas-driven filter pressing from granitoid crystal mush emplaced incrementally in the middle to upper crust and powered by basaltic magma repeatedly injected into the lower reaches of the mush column. Balance between heat loss and basaltic influx determines whether the mush freezes or partially thaws at any given time, blurs internal contacts in resulting plutons, and can produce large volumes of crystal-rich ignimbrite or rapid separation and eruption of crystal-poor rhyolite. Lifetimes of the largest volcano-plutonic systems, such as the Altiplano-Puna or Southern Rocky Mountains volcanic fields, are comparable to the ~10 Myr of the Tuolumne Intrusive Suite.
V43G-02
Caldera-related Tertiary Granitic Plutons in the Southern Rocky Mountain Volcanic Field, Western USA: Links to Subvolcanic Batholiths
Granitic intrusions, ranging from small plugs coring precursor stratocones to composite batholith-scale bodies, are exceptionally exposed within and adjacent to the numerous Tertiary calderas (37-23 Ma) of the Southern Rocky Mountain volcanic field (SRMVF), as result of high regional topography, large-scale faulting associated with the Rio Grande rift system, and associated deep erosion. Such plutons are variable in age, texture, composition, and size relative to associated ignimbrite calderas. Large caldera subsidence structures are direct evidence for batholith-scale magma bodies in the upper crust, for which these plutons are inferred to provide samples of shallow solidified residua. Most exposed plutons are smaller than associated calderas, but large negative gravity anomalies (to –50 mgal) document upper-crustal presence of vertically extensive composite batholiths that are much larger than individual calderas. Some granitic plutons within or near SRMVF calderas are indistinguishable in age from the time of ignimbrite eruption, but others are millions of years younger. Pluton textures range from nearly aphanitic to medium-grained equigranular or megacrystic granitic, reflecting variable cooling and crystallization rates in shallow crustal environments. Compositions of SRMVF intrusions vary from diorite to highly evolved granite; those most closely related in age and location to calderas commonly are compositionally similar to more mafic late-erupted portions of associated ignimbrites. These relations are consistent with interpretation of pluton compositions as mushy residua from prolonged fractionation energized by sustained mafic magma inputs, during which more evolved upper parts of the subcaldera magma chamber were largely erupted as ignimbrite. Sparse recent U-Pb-zircon age data for granitic rocks elsewhere suggest that deeper-level plutons in Cordilleran arcs may have longer crystallization histories than shallower subvolcanic cupolas, contrasts inferred to reflect more prolonged open-system recharge, mixing, and crystallization at greater depths in the batholithic environment. A continuing uncertainty is the proportion of the eventual subvolcanic batholith present at times of peak ignimbrite volcanism, versus continued later- and post-volcanic enlargement.
V43G-03
Characteristics of Young Rhyolites at Taupo, New Zealand: Implications for the Sub-Surface Plutonic System
The young history of Taupo volcano captures the growth and destruction in the 26.5 ka ca. 530 km3 Oruanui eruption of a large rhyolitic magma body, together with the subsequent rejuvenation of magma sources below the volcano. Integration of field information with petrological and isotopic studies at the whole-pumice and single- crystal scales provide a picture of this history. Several important contrasts are inferred to exist between Taupo and comparably-sized, long-lived silicic foci such at Long Valley and in the Bishop Tuff. At Taupo the following are demonstrable. 1. Even in crystal-poor rhyolites like the Oruanui, many grains are inherited antecrysts or xenocrysts. The Oruanui crystal-poor rhyolite body was an open system, with influxes of crystals (plus melt) from remobilised older crystal mush, melted metasedimentary country rocks and plutonics, and crystal-poor basaltic to andesitic magmas. 2. All the Taupo rhyolites were well mixed prior to eruption, and there are no gradients in the eruption products to suggest that the holding chamber(s) were stratified to any extent. 3. Mafic magmas rose into, interacted with, and ponded on the floors of crystal-poor rhyolite in the Oruanui and Waimihia (3.5 ka) examples, again implying that the chamber floor was sharply defined, not a gradual progression down into a more crystal- rich root zone. 4. Pre-Oruanui activity involved contrasting magma types being generated simultaneously, but erupting from geographically separated vents. Post-Oruanui activity has seen (subtly) contrasting magma groups being erupted from vents in the same geographic area, but separated in time. The Oruanui and post-Oruanui magmas are different and do not appear to be related by consanguinity or by mixing – the Oruanui eruption effectively destroyed its magma body. These features are consistent with rhyolite magma generation at Taupo that is exceptionally fast, driven by high fluxes of mafic magmas into a highly heterogeneous crustal melange of metasedimentary and igneous lithologies. There is no voluminous crystal mush body to act as a buffer for the rhyolite generation processes; the eruption frequency at Taupo gives no chance for such a body to develop.
V43G-04 INVITED
The Pulse of Cordilleran Batholith Formation Revealed in the Spatiotemporal Evolution of Large Silicic Volcanic Fields
While the linkage between Cordilleran plutonism and volcanism remains debated, data for large silicic volcanic fields (LSVF) document informative space-time-composition parallels. The comparable dimensions and spacing of centers and similar petrological and geochemical characteristics support the view that, like Cordilleran batholiths, LSVF are composed of multiple periodically constructed magmatic systems. The spatiotemporal record of volcanism at two well-studied LSVF that are geometrically, compositionally, and temporally comparable, the 10 to 1 Ma Altiplano-Puna Volcanic Complex (APVC) of the Central Andes and the 37 to 26 Ma Southern Rocky Mountain volcanic field (SRMVF) of Colorado, provide windows into the construction of the uppermost parts of Cordilleran batholiths. Both the APVC and SRMVF involved eruption of numerous dacite to rhyolite ignimbrites with volumes >100 km3; several with volumes >1000 km3. Some calderas in both regions were polycyclic, with the largest caldera sources 60 km or more across. Area covered in both regions is on the order of 70,000 to 100,000 km2, during eruptive activity of ~10 m.y. Cumulative magmatic volume of ignimbrites is about 10,000 km3 for the APVC and 15,000 km3 for the SRMVF, and estimated peak magma- production rates are as high as 12,000 to 8,000 km3/m.y respectively. Calderas in both areas are associated with pre-and post-caldera andesitic to dacitic lava eruptions. The calderas and other magmatic centers of both areas lie within regional gravity lows that suggest the subvolcanic growth of upper-crustal composite batholiths associated with the silicic volcanism. Both the APVC and the SRMVF lie along the east margins of broad long-lived Cordilleran magmato-tectonic zones, involving plate convergence and low-angle subduction. The APVC is associated with a regional seismic anomaly interpreted as indicating the presence of partial melt in the middle crust; no comparable feature(s) are present beneath the SRMVF, plausibly because of its greater age. The spatiotemporal records from these fields suggest that the combined volcano-plutonic system records a magmatic flare-up that was at least an order of magnitude more intense than the steady-state rates that characterize the long-term evolution of destructive plate margins. The flare-up initiated suddenly, at rates slightly above steady state, escalated to a climactic stage, and declined rapidly. This pattern, and the organization of activity into distinct pulses of increasing intensity and spatial definition with time at LSVF should produce a plutonic record. While Cordilleran batholiths such as the Sierra Nevada Batholith in California contain evidence of comparable flare-ups, resolution of the plutonic record does not distinguish individual pulses. Issues in need of future study include refining the volume-time and spatiotemporal patterns, determining the trigger for magmatic flare-ups and controls on episodic magmatism—both volcanic and plutonic , and resolving the disparate records in the two realms.
V43G-05
The River Mountains Volcanic Section – Wilson Ridge Pluton, a Long Lived Multiphase Mid- Tertiary Igneous System in Southern Nevada and Northwestern Arizona, USA
206Pb/238U zircon dates (LA-ICPMS) from 106-40 μm spots on 49 zircons suggest the Wilson Ridge Pluton in northwestern Arizona and its corresponding volcanic cover in the River Mountains of southern Nevada represent a complex multiphase igneous system active for 4.2 million years (based on a zircon core-rim pair) to a maximum of 7.2 million years (from two zircon rim dates 18.9 ± 0.8 to 13.1 ± 0.6 Ma). This period of activity is significantly longer than the 500 thousand year interval (12.99 ± 0.02 to 13.45 ± 0.02) determined by 40Ar/39Ar sanidine, biotite, hornblende, and whole rock dates. The 40Ar/39Ar dates only reflect the time when the igneous system cooled to mineral closure temperatures during emplacement in the upper crust. Zircon xenocrysts identified in cathodoluminescence images range in age from 1517.5 ± 11.2 Ma to 21.3 ± 0.8 Ma. Inherited zircon cores are as much as 8.9 million years older than their rims. Zircon dates correspond to pluton stratigraphy with late stage dikes at 15.3 Ma (mean age based on 9 dates), quartz monzonite intermediate in composition and age (mean age 15.5 Ma based on 20 dates), and the oldest unit, the Horsethief Canyon diorite (mean age 17.5 Ma based on 6 dates). Although the mean ages correspond to stratigraphy, the spread of ages for each unit overlaps, therefore these correlations are preliminary. The River Mountains volcanic section lies 20 km to the west of the pluton and may have been separated from it by west directed motion along the Saddle Island detachment fault. The River Mountains volcanic section and the Wilson Ridge Pluton are considered a single igneous system as demonstrated by major and trace element geochemistry, whole rock isotopic analyses (Sr and Nd), previous 40Ar/39Ar and K-Ar dates, mafic enclave chemistry, extensive magnesio-riebeckite alteration unique to both the River Mountains volcanic and Wilson Ridge Plutonic sections, and the location of the Saddle Island fault. Preliminary zircon dates from the River Mountains stock (18.1 ± 1.3 Ma to 14.1 ± 0.9 Ma) and River Mountains volcanic section (15.2 ± 0.3 to 13.6 ± 0.7 Ma) fall within the range of dates for the Wilson Ridge Pluton. These overlapping zircon ages provide additional evidence of a link between the volcanic and plutonic sections. Ongoing research includes U/Pb dating of zircons from each of the units within the Wilson Ridge Pluton, and River Mountains volcanic section using a sensitive high-resolution ion probe (SHRIMP).
V43G-06
Enigmatic Connection Between Rhyolites and Shallow Granites in a Cambrian Rift
The Cambrian Southern Oklahoma Aulacogen (SOA) exposes shallow-seated, A-type sheet granites emplaced on top of layered mafic bodies and under cover of A-type rhyolites, this gabbro-rhyolite boundary being a crustal magma trap. Granites and rhyolites appear to overlap in age, although rhyolites built up first, with the rhyolite pile thickening as later granites coarsened texturally to medium-grained types from earlier fine-grained sheets. Outcrop relations + widespread subsurface data, including regional gravity, indicate rhyolite volume exceeds granite volume. Both silicic and mafic members of the SOA sequence have the same positive Nd signature showing a fundamental genetic relationship between the rifting process and the development of new crust in the SOA. Major element geochemistry of rhyolites (~76% SiO2) and granites (73-76% SiO2) is similar, as are many trace element abundances and signatures. However, some indices are different: Zr in rhyolites is ~600-700ppm, in granites ~300-500; Zr/Nb in rhyolites 8-11, in granites 4-8; K/Rb in rhyolites 300's, in granites 200's. In only one locality can a continuous connection between fine-grained granite and a texturally rhyolitic lobe be seen. Thus, it is clear that none of the rhyolites and granites are directly related at their emplacement level. Conclusions: 1)Silicic magma, both rhyolitic and granitic, was fractionated from mafic sources in the middle rift crust. 2)Magma driving pressures for silicic liquids must have been higher for those becoming rhyolites than for those that became granites. 3)These differences imply that rhyolitic magma generally came from a deeper depth in the mid-crust, although the ultimate mafic sources may have been similar. 4)More generally as this case shows, spatially near, and chemically similar, rhyolite and granite bodies in the same tectonic setting may not be directly related.
V43G-07
Igneous origin of K-feldspar Megacrysts in Granitic Rocks of the Sierra Nevada Batholith
Study of the four principal K-feldspar megacrystic granitic plutons and related porphyrys in the Sierra Nevada composite batholith indicates that the included megacrysts are phenocrysts that grew in contact with granitic melt in long-lasting magma chambers. These 89-83 Ma plutons or intrusions are the youngest in the range, and represent the culminating magmatic phase of the batholith. They are the: Granodiorite of Topaz Lake; Cathedral Peak Granodiorite, Mono Creek Granite, Whitney Granodiorite, Johnson Granite Porphyry, and Golden Bear Dike. The zoned megacrysts in each of these igneous bodies attain 4-10 cm in length and all display oscillatory zoning with each zone beginning with a sharp increase followed by a gradual decrease in the concentration of BaO - commonly from 3 to 1 weight percent. Some of the more pronounced zones overlie resorption and channeling features on the underlying zone. Trains of small mineral inclusions (plagioclase, biotite, hornblende, quartz, sphene, and accessory minerals) are parallel to the BaO-delineated zones. The long axes of the inclusions are preferentially aligned parallel to the zone boundaries and inclusions are sorted by size from zone to zone. The growth temperature of sphene included in K-feldspar megacrysts is estimated by use of a Zr-in-sphene geothermometer. The sphene grains all yield igneous temperatures, mainly 735 - 760 °C. Sphene grains in the granodiorite host marginal to the megacrysts range to lower growth temperatures, in some instances into the subsolidus range. The zoning of the megacrysts, their presence in quenched porphry dikes, and the limited range and igneous values of growth temperatures of sphene inclusions within them, support the interpretation that the megacrysts formed as igneous sanidine phenocrysts, and that intrusion temperatures varied by only small amounts while the megacrysts grew. Each Ba- enriched zone was apparently formed by a repeated surge of new, hot melt injected into the large magma chamber represented by the plutons. Each recharge of hot magma offset cooling, maintained the partially molten or mushy character of the chamber, and induced convective currents that would loft settling megacrysts back up into the chamber. These processes continued long enough to provide the necessary conditions for the growth of these extraordinarily large K-feldspar phenocrysts.
V43G-08 INVITED
Questioning the Sedimentary Paradigm for Granites
A critical question regarding volcano-pluton links is whether plutons are samples of magma that passed through on its way to eruption, or residues left behind after volcanic rocks were extracted. A persistent theme of recent work on granites sensu lato is that many are sedimentary accumulations of crystals that lost significant volumes of magmatic liquid. This view is based on observations of structures that clearly seem to reflect deposition on a magma chamber floor (e.g., flows of chilled mafic magma into silicic magma) and on the inference that many other structures, such as modal layering, truncated layering, and crystal accumulations, reflect crystal sedimentation on such chamber floors. There are significant physical and geochemical reasons to question this view, based on observations in the Sierra Nevada of California and similar results from other batholiths. First, few granites show the enrichments in Ba, Sr, and relative Eu that feldspar accumulation should produce. Second, sedimentary features such as graded bedding and cross-bedding form in highly turbulent flows, but turbulence is unachievable in viscous silicic liquids, where velocities on the order of 104 m/s would be required to induce turbulence in a liquid with η=104 Pa s. Third, tabular modally layered domains commonly cut surrounding modal layering on both sides, and orientations of modal layering and of the troughs of "ladder dikes" commonly scatter widely within hectare-sized areas; it is difficult to reconcile these features with gravity-driven settling. Fourth, accumulations of K-feldspar megacrysts are typically inferred to be depositional, but this is precluded by crystallization of most K- feldspar after rheologic lock-up occurs. Finally, accumulations of K-feldspar and hornblende are typically packed too tightly to be depositional. With analogy to layered mafic intrusions, many features attributed to crystal sedimentation in granites may be better explained by crystal aging and other in situ chemical processes. In particular, many of these features may record pore-melt flow paths rather than depositional processes.